When
"How I Met Your Mother" showed us the mother's face in the Season 8 finale, it came as a big surprise to viewers. Almost as big a surprise was the fact that the show managed to keep
Cristin Milioti's casting a secret.

The show's creators have spoken about the
lengths they went to in order to keep the mother's identity under wraps, and Milioti says at times her casting felt like an undercover operation.

"My agent and manager called and said [creators]
Carter [Bays] and
Craig [Thomas] were interested in meeting with me via Skype for a possible arc on 'How I Met Your Mother,'" Milioti tells reporters on a conference call. "But we didn't know what the role was, we didn't know anything about it, and they didn't tell us anything about it. ... It was very, very secret. So I said sure, and I had this wonderful Skype meeting with them where we didn't talk about the show at all. We just talked about what I did when I was growing up and what my hobbies were. I was in 'Once' [on Broadway] at the time, so they asked what it was like to work on a musical. I instantly sort of clicked with them.

"Then I found out I was being flown out there to have a chemistry read with
Josh [Radnor], but I still didn't know what the role was. I didn't know if it was like a red herring or what it was. Then I got out there, and it was pretty clear -- I kind of put it together because it was so secretive. I signed like a Russian novel's worth of confidentiality agreements. The makeup trailer was locked,
Neil Patrick Harris congratulated me by accident. So I had my sneaking suspicions."

After she landed the role, she flew to Los Angeles the day after finishing her run in "Once." "I was in an out-of-body, white-hot panic from pretty much the second the plane touched down to the second my plane took me back to New York," she says. "I was taken [to the stage] on a golf cart, I was in this dark coat, it was by cover of night -- it was really, really awesome and felt like I was some sort of secret assassin. Then everyone welcomed me as the mother on set, and it was wonderful."

After Milioti got the part, Bays and Thomas sent her DVDs of the entire series to date. She says that while it's been "so strange to watch them all knowing I'm the mother," doing so has also helped ease any pressure she may have felt.

"Josh and I were doing an interview on set recently, and the interviewer asked me, 'How are you dealing with this intense pressure of living up to being the mother?' I literally had never thought of it until she said that," she says. "Once my spirit returned to my body, I was like, 'I've never thought about that.' That's not a thing you can have control over. I just go in and do my best, and Carter and Craig won't let anyone down. ... They will not let me let anyone down."

Milioti also picked up a few "factoids" about the mother while watching the show, but she says the hints "HIMYM" has dropped about her character over the years haven't really informed how she plays the character.

"It's kind of all coming from what's in front of me," she says. "I definitely keep things in mind, the little hints and factoids that have been dropped about the character. But the writing is so good that I really just have to do what's in front of me and play with the other cast members. ... It's been a pretty easy integration."

Fans will get to see the mother -- even Milioti doesn't know her character's name yet -- interact with Lily (
Alyson Hannigan) in Monday's (Sept. 23) season premiere, and she says she's also very excited for fans to see some scenes she recently shot with Radnor. That episode is scheduled for early November, and suggests the show will jump ahead in time to show some of Ted and the mother's relationship.

"It will play out in a much more exciting way, I think, than just the 48 hours [of the wedding weekend]," Milioti says of Season 9. "It is going to be so satisfying. That I can say with utmost assuredness. It's going to be amazing."